An air curtain device is one among air jet application devices. Air jet application technology related to the air curtain device will be described.
Each of FIGS. 1(a) and 1(b) shows speed distribution of a free air jet discharged from a nozzle into a static environment according to a non-patent document No. 1. Each of the speed distributions can be divided into a first region neighboring the nozzle and a second region downstream of the first region. The first region neighboring the nozzle is called an initial region, with a jet core present in the center, and the jet core is surrounded by a mixing region, wherein the jet and the surrounding fluid mix with each other to form a mixed flow including vortex flows. The mixing region expands and the jet core region diminishes as the distance from the nozzle outlet increases. Thus, the core region finally terminates. A region downstream of the position where the core region terminates is called a developed region, wherein the mixed flow diffuses.
The jet core shown in FIG. 1(a) is an irrotational parallel flow core, i.e., a parallel flow core which does not include vortex flows, generated by a nozzle formed by a turbulent flow runup zone outlet in a three-dimensional axisymmetric duct shown in FIG. 2. Length X1 of the parallel flow core is X1≈10D.
The jet core shown in FIG. 1(b) is a mixed flow core including vortex flows generated by a two-dimensional nozzle. Length X2 of the mixed flow core is X2≈6D
The parallel flow core of FIG. 1(a) shows strong air current interruption performance because the parallel flow core is an irrotational flow, i.e., a flow which does not include vortex flows. The mixed flow core of FIG. 1(b) shows weak air current interruption performance because the mixed flow is a vortex-including flow.
The mixing region is formed around each of the jet cores shown in FIGS. 1(a) and 1(b) as a result of a large speed gradient between the jet core and the surrounding fluid and an accompanying contribution from viscous fluid flow. An outer periphery of the mixing region forms a jet outer edge. A suction flow is generated on the jet outer edge due to the accompanying action of viscous fluid flow. The suction flow draws surrounding fluid into the mixing region.
FIG. 2 shows a runup zone of turbulent flow in which the parallel flow core of FIG. 1(a) is generated according to a non-patent document No. 2. Mixed fluid flow including vortex flows enters an inlet of the turbulent flow runup zone. A boundary layer of small thickness is generated on a duct wall. As the fluid flow moves downstream, thickness of the boundary layer gradually increases and the vortex flows gradually decrease. When the fluid flow advances by a distance La, La being called runup zone length, the vortex flows dissipate and a parallel flow, i.e. an irrotational flow, with a dish shaped speed distribution is generated. The parallel flow advances in the duct at a steady state speed, while keeping a constant speed distribution. Thus, the dish shaped speed distribution advances in parallel. As described above, the vortex flows exist at the inlet of the runup zone, but dissipate in the runup zone, and thus, the parallel flow not including the vortex flows is generated at the outlet of the runup zone. According to a non-patent document No. 3, the runup zone length La is La≈50D (D is duct breadth when the runup zone is formed by a duct of rectangular cross section and diameter when the runup zone is formed by a duct of circular cross section).
FIG. 3 shows a two dimensional slot nozzle according to a non-patent document No. 4. A jet core screen generated by the two dimensional slot nozzle includes vortex flows and shows weak air current interruption performance. However, the two dimensional slot nozzle is widely used for conventional air curtain devices because it is easily manufactured.
FIG. 4(a) shows contour lines of equal speed flows of a thoroughly developed turbulent parallel flow in a duct of rectangular cross section after passing through a runup zone according to a non patent document No. 5. FIG. 4(b) shows speed distribution of a thoroughly developed turbulent parallel flow after passing through a runup zone according to a non patent document No. 6. As can be seen from FIGS. 4(a) and 4(b), a thoroughly developed turbulent parallel flow in a duct forms an axial speed distribution symmetric to X-X axis and Z-Z axis and forms a parallel flow, i.e., an irrotational flow, in which all speed components have vectors pointing in the same direction.
FIG. 5(a) shows a parallel flow core generated by a nozzle formed by an outlet of a runup zone of rectangular shaped cross section. FIG. 5(b) shows a parallel flow core screen generated by the nozzle formed by an outlet of a runup zone of rectangular shaped cross section when only the ceiling and the floor of the duct are extended from the nozzle. An air curtain device formed by the parallel flow core screen has an advanced feature.